Bone Biology

The major functions of bone are:

  • To provide structural support for the body
  • To protect vital organs
  • To provide an environment for bone marrow
  • To act as a mineral reservoir for calcium homeostasis in the body

How Do Bones Work?

Bone structure is somewhat similar to reinforced concrete that contains structural metal reinforcement rods or bars. These metal reinforcements are commonly called rebar. Protein strands make up the rebar of bone. Calcium and phosphorus mineral crystals deposited around the protein strands are somewhat like the concrete poured around the rebar in reinforced concrete. The protein strands provide the tensile strength that holds everything together and the minerals provide the solid structure. If bones were made only of protein, they would be too flexible. If bones were made solely from minerals, the skeleton would be too brittle.

Bones also are quite different from reinforced concrete. First, bones are not completely solid and more importantly bone is alive! Bone tissue is very dynamic. The structure of bone is constantly being remodeled by cells that break down bone and other cells that rebuild bone. This constant bone remodeling is often called bone turnover. Since bones are able to go through this remodeling process, they can heal when broken. Remodeling continues throughout life. As a consequence, the adult skeleton is fully replaced about every 10 years! Also unlike reinforced concrete, bones are hollow tubes. This design allows them to be strong, yet not too heavy.

Bones Do Much More for the Body than Just Provide Structure

Another critical function of bones is to serve as an important storage reservoir for the mineral calcium. The calcium in the body’s skeleton is both constantly released from bones and taken up by bones during bone turnover. This allows the body to maintain stable levels of calcium in the blood and other body fluids. Many essential body functions (such as heart and nerve function) are highly dependent on carefully regulated levels of calcium in the body.

There are three special types of cells that are found only in the bone.
These cell names all start with “OSTEO” because that is the Greek word for bone.

  • OSTEOCLASTS are large cells that resorb (dissolve) the bone. They come from the bone marrow.
  • OSTEOBLASTS are the cells that form new bone. They also come from the bone marrow and are related to structural cells. Osteoblasts work in teams to build bone. They produce new bone called “osteoid” which is made of bone collagen and other protein. Then they control calcium and mineral deposition. They are found on the surface of the new bone.
  • OSTEOCYTES are cells inside the bone. They also come from osteoblasts. Some of the osteoblasts turn into osteocytes while the new bone is being formed, and the osteocytes then get surrounded by new bone. They are not isolated, however, because they send out long branches that connect to the other osteocytes. These cells can sense pressures or cracks in the bone and help to direct where osteoclasts will dissolve the bone.